Heat-sensitive recording materials record images by using physical or chemical changes that occur to objects due to thermal energy, and a great number of processes have been studied for these materials. A heat-sensitive recording material that uses a physical change of an object caused by heat has long been known as "wax type" heat-sensitive recording paper. This type of paper is currently used for electrocardiograms or the like. Several color forming mechanisms have been proposed for a heat-sensitive recording material which utilize a heat-induced chemical change, and a typical example is known as "two-component color forming system based heat-sensitive recording sheet." This sheet is made by coating a base with a dispersion of fine particles which include two heat-reactive compounds that are separated from each other by a binder or the like. One or both of the compounds are melted so that they contact each other and cause a color forming reaction by which a record is produced. The two heat-reactive compounds are generally called electron donor and electron acceptor compounds. A great number of combinations of these compounds are known. However, they basically consist of those which form a metal compound image and those which form a dye image.
The two-component color forming system based heat-sensitive recording sheet (1) depends on primary color formation and requires no development step, (2) has a texture similar to that of plain paper and (3) is easy to handle. In addition to these advantages, one which uses a colorless dye as an electron donor compound (4) achieves high color density and (5) permits easy manufacture of heat-sensitive recording sheets forming various hues of color. For these reasons, the two-component color forming system based heat-sensitive recording sheet is most commonly used as heat-sensitive recording material.
The heat-sensitive recording sheet having the unique features described above has recently begun to draw researchers' attention as paper that is suitable for recording the received image in facsimile communications. When a heat-sensitive recording sheet is used as recording paper for a facsimile, no development is needed. Accordingly, a facsimile receiver of simplified construction can be used. The fact that the recording paper is the only consumable is advantage with respect to the maintenance of the equipment. However, the use of such a sheet is disadvantageous in that it relies on thermo-recording and therefore has a slow recording speed. The slow recording speed is due to the slow heat response of the thermo-recording head and the heat-sensitive recording material used. With the recent advance in technology, thermo-recording heads having good heat response characteristics have been developed. However, no heat-sensitive recording material that fully meets this requirement has been devised.
Therefore, one object of this invention is to provide a heat-sensitive recording material having good heat response characteristics that enables high-speed recording. More specifically, the invention intends to provide a heat-sensitive recording material which is distinct from prior art recording material that uses a heat pulse of a width of about 5 ms (milli second). It is an object of this invention to produce a material which achieves satisfactory color density with a heat pulse of a width of less than 2 ms (milli second). To achieve this object, the temperature at which color is formed in the heat-sensitive recording material must be decreased. Conventionally, color is formed at a desired temperature by using a colorless electron donor compound (hereunder referred to as a color former) and an electron acceptor compound (hereunder referred to as a developer) at least one of which has a low melting point. We previously proposed in U.S. patent Ser. No. 58,399 and GB No. 2,033,594 A a developer made of a condensate of phenol and aldehyde. Japanese Patent Publication No. 4160/68 teaches the addition of a heat-fusible substance to a combination of color former and developer that forms color at a desired temperature. The heat-fusible substance is added in order to satisfy all other requirements for a heat-sensitive recording material, e.g., white background, long keeping quality of the color forming system, low cost and good hue of color. This substance must be miscible with either the color former or developer or both when it is melted. Because of its purpose, the heat-fusible substance is generally made of a compound that has a lower melting temperature than the color former and developer. However, in most cases, the recording material forms color at a temperature significantly lower than the melting point of the heat-fusible substance. This is perhaps because the heat-fusible substance forms a partial eutectic mixture with the color former or developer and the melting point of the blend is reduced to the eutectic point. For example, in a system wherein the color former is crystal violet lactone (m.p. 178.degree. C.), the developer is 2,2-bis(p-hydroxyphenylpropane) (m.p. 158.degree. C.) and the heat-fusible substance is stearic acid amide (m.p. 140.degree. C.), the recording material forms color at about 80.degree. C. In this sense, the heat-fusible substance need not be a compound whose melting point is lower than that of both the color former and developer, and any compound that causes a reduction in the melting point can be used.
Although the heat-sensitive recording material described above forms color at a desired low temperature, it possesses a disadvantageous characteristic. For instance, a fairly long heating period is necessary for providing satisfactory color density. The reason is that a heat-sensitive recording material containing a heat-fusible substance forms color by going through the following steps: (1) the melting of the heat-fusible substance, (2) the dissolution of the coupler and developer into the heat-fusible substance, and (3) the color forming reaction between the color former and developer; and step (2) governs the rate of the color forming reaction. Therefore, in spite of its satisfactorily low color forming temperature, the heat-sensitive recording material is still unsatisfactory for use as high-speed heat-sensitive recording material that is in increasing demand these days.
In order to solve this problem, a first method involves minimizing the size of the particles of the color former, developer and the heat-fusible substance. By reducing the particle size, the melting and dissolution speeds are increased making high-speed heat-sensitive recording possible. However, a great deal of energy is required to crush the color former, developer and heat-fusible substance into small particles. In addition, the small particles require the use of an increased amount of binder when applying a coating of the mixture onto a base.
The second method, developed from the first method described above, is characterized by forming a homogeneous mixture of the heat-fusible substance with either the color former, developer or both. Specifically, a uniform melt of the color former or developer and the heat-fusible substance is cooled to a solid. Alternatively, the color former or developer and the heat-fusible substance are dissolved in a solvent, followed by evaporation of the solvent or mixing with a precipitation solvent to form a precipitate. This method is very effective in forming a high-speed heat-sensitive recording material because the time required for the color former or developer to dissolve in the heat-fusible substance can be assumed to be almost zero. However, to provide a uniform mixture, the developer and heat-fusible substance must first be melted and then cooled to a solid before it is crushed and even pulverized. Alternatively, the three ingredients must be dissolved in a large quantity of solvent. All of these procedures are unsuitable for practical application. Furthermore, heat-sensitive recording material prepared by these procedures is likely to fog during handling.
We have made various efforts to devise an alternative process for producing a high-speed heat-sensitive recording material and, as a result, have accomplished this invention.
Therefore, another object of this invention is to provide a simple method for producing a developer that melts at a desired color forming temperature allowing it to enter into a color forming reaction with a color former, as well as a heat-sensitive recording material that uses such a developer.
The objects of this invention can be achieved by using as a developer for heat-sensitive recording an organic acid having fusion-bonded thereto a heat-fusible substance having a melting point in the range of from 60.degree. C. to 150.degree. C.